X-ray image converters utilizing luminescent materials have long been known such as radiographic intensifier screens, fluoroscopic screens, and x-ray image intensifier tubes. For example, in U.S. Pat. No. 3,617,743, assigned to the assignee of the present invention, there is disclosed lanthanum and gadolinium oxyhalide luminescent materials activated with terbium which exhibit superior conversion efficiency when employed to convert x-rays impinging on said phosphor medium to visible light. X-rays from a suitable x-ray source which pass through an object and impinge upon said phosphor medium can form an immediate first light image which can be recorded on photographic film as well as produce a radiographic latent image which remains in said phosphor medium unless thereafter recalled by a suitable energy source to produce a second light image by thermoluminescent response. To further illustrate the nature of said latter phosphor behavior, there is disclosed in U.S. Pat. No. 3,996,472 also assigned to the present assignee, various rare earth oxyhalides coactivated with terbium and a second activator selected from zirconium and hafnium exhibiting superior thermoluminescent behaviour in radiation dosimeters when subjected to heat stimulation. In more recently issued U.S. Pat. Nos. 4,346,295 and 4,356,398 there is disclosed laser means emitting light as the source of stimulating energy which produces the same type thermoluminescent response in various phosphor materials. A hafnium-neon laser is therein disclosed as the energy stimulation source for said purpose. Such method of exciting the phosphor materials has been termed "photostimulation" wherein the energy from optical photons (ultraviolet, visible, or infrared radiation is said to stimulate the emptying of stored energy to produce limit emission of the type termed "photostimulated luminescence" as distinct from "thermostimulated luminescence" which is produced by heating. Accordingly, such terms as "thermoluminescent response," "thermoluminescent screen," and "thermoluminescent layer" as used hereafter in connection with the present invention are intended to signify responsiveness to either photostimulated luminescence or thermostimulated luminescence. In a similar manner such terms as "energy source" and "energy" as applied hereafter in describing how said thermoluminescent response is produced in the present phosphor materials shall mean either heat and/or light energy with the term "light" to include ultraviolet, visible, and infrared radiation.
Real time radiographic image systems generally utilize thermoluminescent responsive phosphor materials because the electronic signal processing circuitry does not respond quickly enough to process the first light image produced by x-rays in the phosphor medium. The term "real time" signifies a digital computer being used to enhance the quality of the final radiographic image by various already known information processing techniques. For medical radiography, very short x-ray exposure times of 0.01 to 0.05 seconds are commonly employed. These short exposures present major problems for accurate read-out in real time imaging systems by such devices as vidicon television cameras. Conventional real time imaging systems also lack read-out means capable of recording complete and accurate medical images with resolutions of four lines per millimeter or better thereby further limiting the quality of a digitized light image. The digitizing of said light image requires handling of discrete values called "pixels" with the brightness of each pixel being approximately equal to the average brightness of the corresponding area in the original light image. Since more than one million pixels are required to produce a 10 inch by 12 inch final size digitized image having four lines per millimeter resolution, only photographic film is now used to record these images in real time.
Accordingly, improved thermoluminescent phosphor materials are still being sought for real time radiographic imaging systems especially to improve resolution in the final digitized image. For example, a phosphor with an improved thermoluminescent response such as increased brightness can provide better resolution capability in the digitized image along with still further benefits. Since the radiographic latent image stored in the phosphor medium is recalled by energy application, it becomes further desirable to lower the energy requirements at which the second light image is formed for use in real time radiographic imaging systems. Lower energy requirements for said thermoluminescent response is further useful since the prospects for an incomplete reproduction of the radiographic latent image should be reduced thereby.